As we all know, one way for our eyes to identify a distance is to use relative sizes of objects. Specifically, the farther we are away from an object, the smaller the object appears, and the closer we are away from the object, the bigger the object appears. This “closer-bigger farther-smaller” relationship is called perspective relationship. In photography, perspective relationship′ degree is relevant to a distance between a camera and a shot object, in particular, the farther the distance between the camera and the shot object is, the weaker the perspective relationship is, and the closer the distance between the camera and the shot object is, the stronger the perspective relationship is. A distortion caused by perspective relationship is called perspective deformation.
FIGS. 1A and 1B are schematic diagrams illustrating principles of perspective deformation. As shown in FIG. 1A, when the shot object is a plane object, since distances from the lens to the object are approximately the same, so it is possible to image uniformly without excessive perspective deformation. However, as shown in FIG. 1B, when the shot object is not a plane, for example, a certain part of the shot object has a convex portion toward the lens, since the convex portion is more closer to the lens, then the convex portion located in the middle of the shot image will be magnified.
Nowadays, electronic devices such as mobile phone, laptop, tablet PC and others always have an image acquiring unit like a camera, in order to achieve a shooting function. However, the image acquiring unit of these electronic devices usually requires a close-up shot, so a wide-angle lens is often used to get a wider range of image. For example, when using a self-imaging function of a smart phone, it often needs to shoot the entire face at a close distance, so the smart phone's front camera is often a wide-angle lens. FIGS. 2A and 2B are schematic diagrams illustrating a distortion caused by perspective deformation. As shown in FIG. 2A, in the image shot by using the smart phone's front camera, since the nose tip is closer to the camera, obviously, the nose of the shot object appears larger in comparison to other organs in the face, the face looks quite incoordinated, which seriously affects the self-imaging effect.
Conversely, if a telephoto lens whose focal length is too large is used to shoot the face, then it needs to shoot in a very far place. As shown in FIG. 2B, as the shot object is farther away, the shot image starts to become flat, losing a sense of depth and details, the face looks no third dimension, which is “flat face”.
In order to solve the problem of such distortion caused by perspective deformation, by means of high-grade lens optical design and sophisticated materials, lens manufactured by using an optimized design of lens group and high-quality optical glass (such as fluorite glass) has already appeared, this can make perspective deformation decrease to a very low degree.
However, it is still impossible to completely eliminate perspective deformation by using this method, currently, lens with the highest quality still produces different degrees of deformation and distortion when being tested under extremely strict conditions. Moreover, the lens manufactured with such high-quality optical glass will lead to a great increase of cost, especially it is hard to gain popularity in electronic devices that do not give priority to the photo shooting function, like smart phone, laptop, tablet PC and others.